I have seen people run Board Game tournaments in the past, but what I want to do is more like a League run over the course of 3-4 months. Generally the games we play are:

Small WorldThunderstoneDominionNightfallSettlers of CatanCutthroat Caverns

Im trying to figure out the best way to do this, considering that one day we will have players A, B, & C. Then the next day we will have players A, C, & D. Basically the players change from day to day. I was thinking about just keeping track of points, (since all of the games listed above) use points to determine the winner. But can't think of a fair way to do it since player A & B are almost always playing, but B, C, D, and E, vary.

You could do it based on point percentage, with a minimum threshold for number of games played. Or you could give out an extra point (or points) for every 10 games played during the season, to encourage and reward participation.

EDIT: Changed win % to point %. If winning was 3 points, second was 2, third was 1, and fourth was 0, and a player played 10 4 player games and earned 21 points, then his average points per game is 2.1.

How about maintaining a chart that has each player along one axis and each game along another. When a game is started where each player is so far unscored in that game, the game becomes a ranked match, recording scores to the chart when done. If one or more players already have a score, just play for fun like usual. Repeat until the chart is completely filled out as players B, C, D and E have time.

Granted this can cause some imbalance since each player will not have played each other player. It'd be a bit more to setup, but you could do similar with the chart including spaces for each player vs player @ game combination (yay 3 axis chart) and track it until every possible combination has been played.

Off the top of my head....I'm sure there are other ways (probably easier/better)...

You could do a "rotissere baseball" style scoring where you do ranks for each game based on all the plays. Highest ranked player gets 6 points, next is 5, down to 1 for the lowest rank (this is assuming you have six players). Total points across all games is the winner.

Within each game you could take average finish to allow for different number of plays (maybe drop best and worst result for each player before taking the average - forces them to at least play three times instead of just playing once, finishing first and then skipping that one for the rest of the league).

You would need to account for different number of players for games like Small World - how do you score a five player game versus a two player game? The math on this is probably better done by someone else.

If some people play far less often than others, there's an old problem you maybe need to watch out for. If you do any kind of straight average (cumulated points divided by number of games played), then you run a big risk that a player attends once, wins, scores the maximum, never plays again, and sits at the top forever with an unbeatable maximum.

The "standard" way round this is: cumulated points divided by games-played-plus-one. We use that in one of our leagues, here at London Board Games Club, where we have leagues on three games, and a "ladder" on a fourth, which is a two-player game.

You say you want to create the league across several different games at once, ongoing. Obviously, you first have to make the different games more equal, rather just take the points scored in-game - I mean, since some games have victory points up to 160 pts, and a different game maybe usually is won by the guy who reaches 20 vp's?

One idea to equalise each game is: 4 players: winner gets 5 league pts, second gets 3 league pts,, third 2, last 1. If 5 players, then winner gets 6, second 4, third 3, fourth 2, fifth 1. Etc.

If your club preference is to reward winning by a bigger differential than that, then maybe add one, i.e. (4 players) winner 6, second 3, third 2, last 1. And 7,4,3,2,1 if 5 players, 8,5,4,3,2,1 if 6 players.

Or even a bigger difference: 7,4,2,1 and 8,5,3,2,1 or so.

If you take up a game that doesn't end in a clear order of 1st, 2nd, 3rd etc, just agree in advance, eg 5 pts for the winner, 1 pt for all losers! (It's nicer to score at least 1 !)

BTW, if you spread the scores more (eg more of a difference for the winner), then you might be better off with cumulated-points divided by games-played-plus-TWO ... or even THREE.

In our Team RoboRally, cumulated points divided by games-played-plus-one has worked well for four years, with 43 names in the league, and 4 different annual champions. But on King Of Tokyo we use divided by games-played-plus-ten. The leagues definitely add interest and banter.